This invention relates to image processing where appearing and vanishing objects are detected in input images, and more particularly to a method, an apparatus and a video surveillance system for generating backgrounds.
A conventional method for detecting a moving object such as a car or a person in input images obtained by means of a video input apparatus such as a fixedly installed video camera, has widely employed the subtraction method where moving objects are detected by comparing a base image for reference with an input image. According to the conventional method, either an input image taken before a particular input image in input video sequence is chosen as the base image for the particular input image, or a separately prepared background is chosen as the base image.
In these methods for detecting moving objects based on the subtractive processing of the input image and the base image, if the subtraction process is performed using the base image selected from among any previous input images, a “false positive” may be caused when the previous input image contains a moving object therein. In such an image environment as security surveillance or intelligent transport system (ITS) where traffic flow is relatively heavy, the selection of input image which contains no moving object is very difficult. Moreover, in such an imaging environment, illumination often and easily varies due to weather change. Accordingly, there may arise a possibility of causing a “false positive” or a “false negative” unless the base image can follow up the time-sequential change in the imaging environment.
A method for generating a background which contains no moving object and which can follow up the changing image environment, is disclosed in, for example, JP-A-9-81714, wherein the background is produced by using a recursive filter in such a manner that a plurality of image sequence (time-sequential images) containing moving objects or illumination changes are subjected to moving average, or by accumulating a plurality of image sequence whereby an image to serve as a background is estimated through statistically processing the accumulated images.